Changes in the mechanical properties of the longitudinal and circular muscle tissues of the rat myometrium during gestation.

Changes in the mechanical properties of the longitudinal and circular muscle tissues of the rat myometrium during gestation were investigated. In isolated longitudinal and circular muscles of the rat myometrium, spontaneous contractions and contractions per unit cross-sectional area induced by 128 mM K+ and 1 X 10(-5) M acetylcholine (ACh) increased with the progress of gestation. These increases appeared in longitudinal muscles to a greater extent than in circular muscles. ACh induced the largest contraction for both intact muscle tissues, at all stages of gestation. In both muscle layers, the ACh-induced contraction reached the same amplitude as the 1 X 10(-5) M Ca-induced contraction of skinned muscles, except for the longitudinal muscle at the 22nd day of gestation. In Ca-free solution containing 2 mM EGTA, ACh produced contraction in both intact muscle tissues at all stages and the amplitude was increased during the progress of gestation, whereas the K-induced and spontaneous contractions ceased. In saponin-treated skinned muscles of both layers, the free Ca concentration required to produce contraction was lowered, the maximum amplitudes of the contraction were increased and the pCa-tension relationships shifted to the left during the progress of gestation. The results indicate that during the progress of gestation, Ca sensitivity of of the contractile proteins and mechanical responses to agonists increased and that the properties of the intracellular Ca store site were also altered in both muscle layers.